void main(List<String> args) {
  var name = 'zhangsan';
  print(name);
  name = 'lisi';
  print(name);
  // 常量
  const BASE_URL = 'https://www.baidu.com';
  print(BASE_URL);
  // 常量不能修改
  // BASE_URL = 'aaa';
  print(BASE_URL);
}